Jonathan Ernst / Reuters

Winning the Democratic presidential nomination should not be easy. Before South Carolina and Super Tuesday, Bernie Sanders had two ways to prevail: the good, long path and the bad, short path. The latter path—finishing first in primaries by holding on to his base on the left while several other candidates divided up the moderate vote—has now closed. The good path—to piece together an outright majority one way or another—has become much more difficult now that the field has thinned. But if Sanders is willing to take the steps necessary to win this way, it would be healthy for him and the party alike.

For supporters, the case for Sanders’s electability goes like this: His policy proposals and he personally are broadly popular, he can expand the electorate by appealing to disaffected young and working-class voters, and he won’t lose much support among Democrats, because the party is united in its eagerness to vote Donald Trump out of office.

Only two weeks ago, Sanders appeared to be the overwhelming favorite to secure the nomination. But that wasn’t a sign of majority support; it was because of how a quirk in the party’s rules for awarding delegates interacted with the front-loaded primary calendar and the large number of candidates. To qualify for delegates at the Democratic National Convention, party rules specify that a candidate must earn 15 percent of the vote within a congressional district. If a candidate receives anything less, those votes are wasted, and all delegates are apportioned among the candidates who crossed the 15 percent threshold. These rules encourage a robust nomination battle when only two or three serious candidates are competing. But in a race with seven or more candidates, the rules favor the one with the most dedicated supporters—even if they don’t come close to making up half the voters.

After the Iowa, New Hampshire, and Nevada contests, Sanders was poised to capture 70 to 80 percent of California’s enormous haul of delegates while earning only a third of the votes. He might have won an insurmountable delegate lead without ever showing he could build a majority coalition within the party. Of course, we all know what happened next. The field cleared at the eleventh hour for Joe Biden after his South Carolina victory. Tom Steyer, Pete Buttigieg, and Amy Klobuchar all dropped out of the race in quick succession. Buttigieg and Klobuchar joined dozens of party leaders in endorsing Biden, making the former vice president the clear anti-Sanders candidate. Biden won 10 of 14 Super Tuesday states. Votes are still being counted in California, and plenty of early votes were effectively wasted on candidates who dropped out or failed to qualify for delegates, but Sanders’s bad path—the short path—to the nomination has been foreclosed.

The good path to the nomination for Sanders is one that forces him to deliver the popular movement that he and his supporters believe he can inspire. His detractors see little evidence that Sanders is bringing new voters to the party, and they fear that his rhetorical broadsides against the Democratic establishment will depress enthusiasm among core Democratic constituencies. They wonder whether he can withstand the negative advertising and skeptical media coverage that general-election campaigns bring. They worry that Baby Boomers—the most reliable voters—will panic when the election is framed as a choice between socialism and capitalism.

But if Sanders in fact can generate record primary turnout among young voters, that would be evidence that he could expand the electorate in November. If Sanders’s popularity remains high when faced with a negative ad blitz financed by Michael Bloomberg or some other wealthy donor, then Democrats can be more confident that he’ll survive a barrage of Republican ads. If Sanders can steadily win converts among core Democratic constituencies, then party members can be more confident that the party will enthusiastically unite around him in November.

The question to ask now is how Sanders will pursue this harder, better path. He could treat the 2020 race as a replay of the 2016 contest, with Joe Biden standing in for Hillary Clinton as the establishment figure too cozy with billionaires. But this strategy is unlikely to work well. As we saw this week, Democratic voters have grown wistful for the Barack Obama years. The 2020 primary is defined by Democrats’ desperation to end Trump’s brazen corruption, cruelty, and incompetence and start repairing the damage he has caused.

Sanders will likely fare better by drawing a contrast with Biden’s own foibles. The former vice president is on his own bad path to the nomination. Political handicappers had good reason to count him out after poor performances in Iowa and New Hampshire. He has not built a campaign apparatus that can win in the fall. A lot of his debate performances and campaign stops have elicited more cringes than cheers. Many would now crown Biden the presumptive nominee, without him demonstrating that he has the agility and acuity as a campaigner to thrive in a long, grueling general-election campaign. He needs a thorough test of his own: He has to withstand months of scrutiny as the front-runner, and make sure voters and party leaders don’t develop a case of buyer’s remorse. Had Elizabeth Warren stayed in the race, she would have forced Biden and Sanders to sharpen their campaigns—and made a credible nominee should either or both of them falter. With her withdrawal yesterday, Biden and Sanders may both be tempted to assume they can win votes as the lesser of two inadequacies.

Especially because the election is likely to be close, the Democratic Party courts catastrophe by choosing its candidate based on a technicality. Democrats should be glad that Sanders can no longer seize the nomination without thoroughly making the case for his candidacy. They should hope that Biden is forced to prove his mettle through hard-fought contests in the months to come as well. This year, the shortest path to becoming a presumptive nominee is also the riskiest.

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